13.4 MS Windows Opportunistic Locking and Caching Controls
There is a known issue when running applications (like Norton Anti-Virus) on a Windows 2000/XP workstation computer that can affect any application attempting to access shared database files across a network. This is a result of a default setting configured in the Windows 2000/XP operating system known as opportunistic locking . When a workstation attempts to access shared data files located on another Windows 2000/XP computer, the Windows 2000/XP operating system will attempt to increase performance by locking the files and caching information locally. When this occurs, the application is unable to properly function, which results in an " Access Denied " error message being displayed during network operations.
All Windows operating systems in the NT family that act as database servers for data files (meaning that data files are stored there and accessed by other Windows PCs) may need to have opportunistic locking disabled in order to minimize the risk of data file corruption. This includes Windows 9x/Me, Windows NT, Windows 200x, and Windows XP. 
If you are using a Windows NT family workstation in place of a server, you must also disable opportunistic locking (oplocks) on that workstation. For example, if you use a PC with the Windows NT Workstation operating system instead of Windows NT Server, and you have data files located on it that are accessed from other Windows PCs, you may need to disable oplocks on that system.
The major difference is the location in the Windows registry where the values for disabling oplocks are entered. Instead of the LanManServer location, the LanManWorkstation location may be used.
You can verify (change or add, if necessary) this registry value using the Windows Registry Editor. When you change this registry value, you will have to reboot the PC to ensure that the new setting goes into effect.
The location of the client registry entry for opportunistic locking has changed in Windows 2000 from the earlier location in Microsoft Windows NT.
You can also deny the granting of opportunistic locks by changing the following registry entries:
HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\MRXSmb\Parameters\ OplocksDisabled REG_WORD 0 or 1 Default: 0 (not disabled)
HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\LanmanServer\Parameters EnableOplocks REG_DWORD 0 or 1 Default: 1 (Enabled by Default) EnableOpLockForceClose REG_DWORD 0 or 1 Default: 0 (Disabled by Default)
To force closure of open oplocks on close or program exit, EnableOpLockForceClose must be set to 1.
An illustration of how Level2 oplocks work:
13.4.1 Workstation Service Entries
\HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\LanmanWorkstation\Parameters UseOpportunisticLocking REG_DWORD 0 or 1 Default: 1 (true)
This indicates whether the redirector should use opportunistic-locking (oplock) performance enhancement. This parameter should be disabled only to isolate problems.
13.4.2 Server Service Entries
\HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\LanmanServer\Parameters EnableOplocks REG_DWORD 0 or 1 Default: 1 (true)
This specifies whether the server allows clients to use oplocks on files. Oplocks are a significant performance enhancement, but have the potential to cause lost cached data on some networks, particularly wide area networks.
MinLinkThroughput REG_DWORD 0 to infinite bytes per second Default: 0
This specifies the minimum link throughput allowed by the server before it disables raw and opportunistic locks for this connection.
MaxLinkDelay REG_DWORD 0 to 100,000 seconds Default: 60
This specifies the maximum time allowed for a link delay. If delays exceed this number, the server disables raw I/O and opportunistic locking for this connection.
OplockBreakWait REG_DWORD 10 to 180 seconds Default: 35
This specifies the time that the server waits for a client to respond to an oplock break request. Smaller values can allow detection of crashed clients more quickly but can potentially cause loss of cached data.